1. Field of the Invention
This invention pertains generally to a hydrogen fuel cell electrical power generating system, and more particularly to an open cathode proton exchange membrane (PEM) system.
2. Description of Related Art
FIG. 1A illustrates a prior art fuel cell system 10 having a fuel cell stack 12 comprised of a plurality of individual fuel cells 26 connected in series (shown in further detail in FIG. 1B). A stack fan 15 is used to provide process oxidizer (air) and also perform a cooling function by either drawing air 22 through cooling features of the stack 12 and delivering air to the cathode, or by blowing air through the fuel cell stack 12 for cooling and delivering air to the cathode. Additionally, there may be ducting 14 to assist in directing the air flow 22 associated with the fuel cell stack 12. In these systems, a fuel source 32 of hydrogen (or optionally reformate) is provided to stack 12. Inlet fuel pressure control can be provided by a pressure regulator 19. The fuel is fed into the fuel cell stack 12 through a fuel inlet valve 16 and exits the fuel cell stack 12 through fuel exit valve or purge valve 28.
The fuel can be delivered by the pass through method or the periodic purge method. In the pass through method, the fuel 32 is continuously bled through the fuel cell stack 12 by way of the fuel inlet valve 16 and the fuel exit valve 28 to prevent the accumulation of inert species such as nitrogen and water vapor in the anode chamber.
In the periodic purge method, the fuel exit valve 28 is held closed while fuel 32 is delivered to the fuel cell stack 12 thought the fuel inlet valve 16. Over time, inert species such as nitrogen and water vapor accumulate in the anode chamber and impede the electrochemical reaction due to the interference of the mass transport of hydrogen to the anode electrodes. This necessitates the periodic opening of the fuel exit valve 28 to purge the inert species from the anode chamber.
Electrical power is taken from the fuel cell stack 12 through electrical leads 18 and 30 and delivered through a switch 24 to an electrical service load 20. Shutting down the system may simply entail disconnecting the electrical service load 20 by opening the switch or relay 24, or discontinuing the supply of fuel to the fuel cell stack by closing the fuel supply valve 16 and opening the purge valve 28 to allow the pressure within the anode volume to equilibrate and then turning off the stack fan 15, stopping the flow of air 22.
This procedure leaves fuel within the anode volume, which allows the electrochemical reactions to continue within the fuel cell stack and creates a potential across the fuel cell stack, a potentially unsafe condition. Leaving the purge valve 28 open allows the anode volume to eventually fill with air, thus reducing the potential across the fuel cell stack 12 to zero, inerting the fuel cell stack 12 and eliminating the unsafe condition.
However, starting and stopping proton exchange membrane (PEM) fuel cell 12 is often detrimental to the platinum catalysts (not shown) used in PEM fuel cells, because high cathode potentials develop during the exchange of oxidizer (air) and hydrogen in the anode volume during the starting and stopping processes. These high cathode potentials cause the corrosion (oxidation) of the carbon catalyst support material on the cathode, leading to the degradation of catalyst itself and a resultant loss of performance.
In addition, when simply opening the purge valve and allowing air to be drawn into and through the fuel cell stack, the anode volume is placed in a mixed gas condition for an extended period of time, leading to very rapid cathode catalyst degradation.
Accordingly, an object of the present invention is a fuel cell system that reduces the residence time of the mixed gas condition at the anode, and reduces the potentials across the fuel cell during the starting and stopping of the fuel cell system, to thereby reduce the impact of mixed gas high cathode potentials. At least some of these objectives will be met in the description provided below.